The following is related generally to optical or fiberoptic components used in optical communication networks and, more specifically, to reducing polarization, wavelength, and temperature dependent loss in fiberoptic components.
Fiberoptic components such as Variable Optical Attenuators (VOAs), optical switches, and tunable optical filters are widely deployed in optical networks, typically in the 1550 nm or 1310 nm wavelength windows, as well as other wavelength ranges. In wavelength-division-multiplexed optical networks where multiple wavelengths are used, so that multiple channels of information can be transmitted or carried on a single fiber, Variable Optical Attenuators are used at various points in the network, to manage the optical power of the multiple optical signals or wavelengths. Optical switches are used to redirect or re-route signals that are transmitted or carried on fibers, by establishing connections between fibers. Tunable optical filters are used to select specific wavelengths or wavelength ranges, and may also be used to scan multiple wavelengths in channel or fiber monitoring applications.
Optical beam-steering technologies of various kinds are often used to implement fiberoptic components such as VOAs, optical switches, and tunable optical filters. For example, MEMS (Micro-Electro-Mechanical Systems) tilting mirrors are often used to steer light from one or more input ports or fibers of a fiberoptic component, towards one or more output ports or fibers. In a MEMS-based VOA, the beam is steered toward an output port, and the degree of alignment of the beam to the output port determines the amount of attenuation. In a MEMS-based optical switch, the intent is usually to have minimal insertion loss, as the beam is steered to the desired output port. Similarly, in a tunable optical filter, the intent is usually to have minimal insertion loss of the selected wavelength or wavelength range, as it is steered to the output port. Also, in the case of some tunable optical filters, the coupling of light to the output port and the geometry of the optical path, serve to determine the shape and width of the selected wavelength's passband.
In fiberoptic components that make use of beam-steering, the coupling of light from the one or more input ports or fibers, to the one or more output ports or fibers, depends on many factors, including the configuration and design of optical elements in the path between the input and output ports, as well as the coupling of the steered beam to the output port(s) or fiber(s). The loss through the fiberoptic component may be dependent on the polarization of the input light, wavelength, and even the ambient temperature. In the case of tunable optical filter components, the coupling of light to the output port(s) or fiber(s) may also determine the shape and width of the filter's passband. The reduction of polarization dependent loss (PDL), wavelength dependent loss (WDL), and temperature dependent loss (TDL) has great value to the designers and implementers of fiberoptic networks. Similarly, improvements to the passband characteristics of tunable optical filter components, such as providing greater isolation of adjacent wavelength channels, also has great value.
In many of the optical network applications of Variable Optical Attenuators, as well other fiberoptic components, it is often necessary or desirable to monitor the optical power of the signal, either on the input side of the component, or (more typically) on the output side. For this reason, it is common practice to use an optical tap and an optical power detector, at either the input or output of an optical component or function. The optical tap splits off a small portion of the optical signal. The split-off optical signal is then directed to an optical detector device, which converts the optical power to an electrical signal, from which the optical power of the signal can be determined. The remainder of the optical signal (the portion that was not split off and directed to the detector circuit) is than passed on to the rest of the network. The portion of the optical power that was split off by the optical splitter, or tap, represents a source of insertion loss to the desired/intended optical signal. Consequently, optical systems could benefit from improvements in providing a tap function for monitoring purposes.